In the field of data storage, there are two main types of storage elements. The first type is volatile memories, in which information is stored in a particular storage element and the information is lost the instant the power is removed from the circuit. The second type is non-volatile memories, in which the information is preserved even when the power is removed. Typically, the manufacturing techniques used to form non-volatile memories are different from standard logic processes.
Non-volatile memories further include one-time-programmable (OTP) memories and flash memories, which can be programmed and erased more than once. Traditionally, fuses or anti-fuses are used as OTP memories. Recently, metal-oxide-semiconductor (MOS) devices are increasingly used as OTP memories, wherein charges are stored in MOS devices to represent states.
FIG. 1 illustrates a conventional memory cell used as a one-time-programmable memory cell. The memory cell includes two gate spacers, each including a storage layer 8, which is separated from substrate 2 and gate electrode 4 by tunneling layer 6. The charges stored in storage layer 8 affects the threshold voltage of the memory cell. Therefore, by determining the threshold voltage of the memory cell, the state of the memory cell can be determined.
For OTP memories cells to represent the state correctly, a reasonable program/erase window, which is the difference between a threshold voltage of a memory cell having charges stored in storages and a threshold voltage of the memory cell not having charges stored in storages, is needed. If the program/erase window is too low, the reliability of the memory cell will be adversely affected. A memory cell provided by the present invention addresses this requirement.